1. Field of the Invention
The present invention relates to light-emitting circuit boards and light-emitting display devices. More specifically, the present invention relates to a light-emitting circuit board and a light-emitting display device, which is preferably used in active matrix electroluminescent display devices.
2. Description of the Related Art
With diversification of information processing devices, demands for flat display elements which consume electric power lower than that of a commonly used cathode ray tubes (CRT) and which can be thinned have been increasing, recently. Examples of such flat display elements include a liquid crystal display element and an electroluminescent element (hereinafter, also referred to as “EL element”). Particularly for organic EL elements, research and development have been actively carried out because such organic EL elements have characteristics such as driving at low voltages, all-solid state, high responsiveness, and self-emitting property.
The organic EL elements are broadly classified into passive matrix type (PM type) and active matrix type (AM type) (for example, refer to Japanese Kokai Publication No. 2000-173779 and Japanese Kokai Publication No. 2002-32037). The passive matrix organic EL elements are driven by a line sequential driving method, and light is emitted only from pixels whose scanning electrodes are selected. Therefore, a display capacity is increased in order to obtain a high luminance. That is, a large instantaneous electric power needs to be applied to each pixel, with an increase in the number of scanning electrodes. Accordingly, the PM organic EL elements have room for improvement in that the power consumption is large and in that the product lifetime is short because deterioration of the light-emitting layer, attributed to application of a large electric power, is significant.
In the AM organic EL elements, an active element is formed inside each pixel, and particularly, a system in which a thin film transistor (TFT) is formed in each pixel has been widely used. According to the AM organic EL elements, for example, in a system in which a TFT is formed inside each pixel, switching can be performed in every pixel. Therefore, according to the AM organic EL elements, the number of the scanning electrodes is not limited in principle and display during almost 100% of one frame period can be performed. Further, the instantaneous electric power can be more suppressed in comparison to the PM organic EL elements. Therefore, such AM organic EL elements permit high luminance, high image quality, and large-capacity display. Further, according to the AM organic EL elements, there is no need to apply a large electric power to each pixel in order to obtain a high luminance, unlike the PM organic EL elements. Therefore, a lower driving voltage and a longer product lifetime are permitted. Therefore, research and development especially for the AM organic EL elements have been actively carried out, recently.
FIG. 4 is a cross-sectional view schematically showing a conventional bottom emission organic EL element. The organic EL element includes a board 2a, a light-emitting organic layer 12 formed on the board 2a, and a first electrode 10 and a second electrode 11 arranged to interpose the light-emitting organic layer 12 therebetween. The first electrode 10 has a function of injecting holes into the light-emitting organic layer 12. The second electrode 12 has a function of injecting electrons into the light-emitting organic layer 12. The holes and the electrons, injected from the first electrode 10 and the second electrode 11, respectively, recombine in the light-emitting organic layer 12, and thereby the light-emitting organic layer 12 emits light. The board 2a and the first electrode 10 are configured to have a light-transmitting property. The second electrode 11 is configured to have a light-reflecting property. Light emitted from the light-emitting organic layer 12 is transmitted through the first electrode 10 and the board 2 to be output from the organic EL element.
If the organic EL element is driven by the active matrix (AM) system, an active matrix (AM) board including TFTs and electrodes is used as a board. FIG. 5 is a cross-sectional view schematically showing one example of a conventional AM organic EL display. As shown in FIG. 5, according to a conventional AM organic EL display, a TFT 17 for driving each pixel, a first electrode 10, a second electrode 11, a flattening film 13, and the like, are formed on a board 2.
The TFT 17 includes: an island semiconductor layer 15; a gate insulator 18 formed thereon; a gate electrode 14 insulated from the island semiconductor layer 15 by the gate insulator 18; and source/drain electrodes 16 which are formed on the island and semiconductor layer 15 and are connected to source/drain regions of the TFT 17.
The first electrode 10 is formed at a plurality of positions on the board 2 in a matrix pattern. The plurality of the first electrodes 10 constitutes pixel regions 1 of the organic EL element, respectively. An insulating film 21 is formed on the first electrodes 10 and provided with an opening only at a light-emitting portion. Each of the first electrodes 10 is connected to the source/drain electrodes 16 of the TFT 17 through an in-pixel contact 20 formed in the flattening film 13, and has a function of injecting holes into the light-emitting layer 12 in accordance with a signal input from the TFT 17.
The second electrode 11 is formed to cover the light-emitting organic layer 12 and a barrier insulating layer 9 and has a function of injecting electrons into the light-emitting organic layer 12. The second electrode 11 is connected to the source/drain electrodes 16 formed in the AM board through an off-pixel contact 3. The source/drain electrodes 16 are formed in the same layer as a layer where a wiring 5 for connecting the in-pixel contact 20 to a terminal for supplying an electric signal and/or an electric power from an external circuit to the AM board is formed.
FIG. 6 is a planar view schematically showing one example of a conventional AM organic EL display.
As shown in FIG. 6, according to a conventional AM organic EL display, a pixel region 1 where a thin film transistor, a first electrode, a light-emitting layer, and a second electrode are stacked on an AM board in this order is formed. In addition, a signal line 4 for connecting a driver circuit 19 including the thin film transistor (containing a source driver circuit 7 and a gate driver circuit 6) to a pixel, a wiring 5 connected to the second electrode through an off-pixel contact 3, and an external terminal 8 connected to the driver circuit 19 and the wiring 5, are formed. An electric signal and/or an electric power supplied from an external circuit pass/passes through the external terminal 8 into the driver circuit 19.
According to the AM organic EL displays, TFTs and electrodes, which are made of silicon with a low light transmittance, need to be formed on the board. Accordingly, the AM organic EL elements have room for improvement in that a ratio of a light-emitting area to a pixel area (opening ratio) is small. Particularly in organic EL elements driven by a current driving system in which variation of display among pixels can be prevented and decrease in display performances, attributed to deterioration of a light-emitting material, can be effectively suppressed, much more TFTs need to be formed inside each pixel, in comparison to the organic EL elements driven by a voltage driving system. Therefore, the organic EL elements in the current driving system have room for improvement in that the opening ratio is further reduced.
For this problem, a top emission organic EL element, in which the second electrode is made of a light-transmitting material and the first electrode is made of a light-reflecting material, and thereby light emitted from the light-emitting organic layer is output not from the first electrode side that is a side of the board where TFTs and electrodes not transmitting light are formed but from the second electrode side, was proposed (for example, refer to Japanese Kokai Publication No. 2004-127551).
According to the top emission organic EL elements, light emitted from the light-emitting organic layer can be output not through the board where TFTs and electrodes not transmitting light are formed. Therefore, the opening ratio can be larger than that of the bottom emission organic EL elements, and thereby organic EL elements with a higher luminance can be provided.
According to the top emission organic EL elements, light emitted from the light-emitting layer is output from the second electrode side, and therefore the second electrode is made of a material with a high light transmittance. Examples of such a material for the second electrode include indium tin oxide (ITO) and indium zinc oxide (IZO). However, electrode materials with a high light transmittance such as indium tin oxide (ITO) have a higher electric resistance than that of metal materials such as silver (Ag) and aluminum (Al), which have been conventionally used as a material for the second electrode. Therefore, the second electrode made of indium tin oxide (ITO) and the like, having a high light transmittance, has a high electric resistance. If the second electrode has a high resistance, voltage reduction occurs at part of the second electrode. Accordingly, the organic EL elements including the second electrodes made of indium tin oxide (ITO) and the like, having a high electric resistance, have room for improvement in that a voltage is not uniformly applied to the second electrode and therefore, uneven image is displayed. In addition, the top emission organic EL elements have room for improvement in that the driving voltage is high because the second electrode has a high electric resistance.
For this problem, a configuration in which a part electrically connecting the second electrode to a substrate-side wiring (off-pixel contact) is formed at a plurality of positions, for example, at four corners of a pixel region was proposed, as a measure for suppressing the voltage reduction generated in the second electrode, thereby uniformly applying a voltage to the second electrode (for example, refer to Japanese Kokai Publication No. 2001-109395). According to this, the voltage reduction generated in the second electrode can be suppressed, in comparison to the case where one off-pixel contact is formed. As a result, a voltage can be uniformly applied to the second electrode.
However, the number of the wiring for connecting the off-pixel contact to the external terminal is increased, with the increase in the number of the off-pixel contact. As a result, as shown in FIG. 7, a space where a wiring 5 for connecting each off-pixel contact 3 to an external terminal 8 is formed is needed, and therefore a frame region (a space between a side of the board and a pixel region) is extended. In this point, there is room for improvement.